1. Field of the Invention
This invention relates to a polycrystalline silicon photoelectric conversion device and a process for its production. More particularly it relates to a polycrystalline silicon photoelectric conversion device having a good energy conversion efficiency, and a process for its production.
2. Related Background Art
For use as an energy source for various equipment and as power sources connected with commercial power grids, solar cells serving as photoelectric conversion devices are the subject of much research.
Solar cells make use of pn junctions in their functional parts, and Si is commonly used as the semiconductor constituting the pn junctions. From the viewpoint of efficiency of converting light energy into electromotive force, it is preferred to use monocrystalline silicon. From the viewpoint of achievement of larger area and lower cost, amorphous silicon types are considered advantageous. In recent years, use of polycrystalline silicon has been studied for the purposes of achieving a low cost comparable to that of amorphous silicon types and a high energy conversion efficiency comparable to monocrystalline silicon types. In processes hitherto proposed, however, bulk polycrystal ingots are sliced into sheet members, and hence it is difficult to make their thickness not larger than 0.3 mm. Thus, they have too large a thickness to absorb light in a sufficient amount, resulting in unsatisfactorily effective utilization of materials. In other words, in order to achieve cost reduction, they must be made to have a much smaller thickness.
Accordingly, it has been attempted to form thin films of polycrystalline silicon by using thin-film forming techniques such as chemical vapor deposition (CVD), according to which, however, the crystal grain size is equal to the film thickness and can only be several-hundredths to scores of microns. Thus, under existing circumstances, such films have a low energy conversion efficiency also when compared with the products produced by slicing bulk polycrystalline silicon ingots.
Under such circumstances, it is also attempted to irradiate polycrystalline silicon thin films with laser light to melt them to effect recrystallization so that the crystal grain sizes are enlarged. This, however, cannot achieve satisfactory cost reduction and makes it difficult to stably carry out the manufacturing process. It has been proposed to form crystalline silicon films on low-cost substrates by zone melting recrystallization (ZMR) in a thickness large enough to absorb sunlight. Hamamoto et al., "Crystal Defects and Solar Cell Characteristics in Polycrystalline Si Thin Films", The Third Workshop on High-Efficiency Solar Cells in Toyama, 1992, p.20.
FIG. 5 cross-sectionally illustrates a solar cell fabricated by the above process. On a metallurgical grade silicon substrate 502, which is an inexpensive substrate with a low purity, an SiO.sub.2 insulating layer 503 as an impurity barrier, a poly-Si (polycrystalline silicon) film 507, and an SiO.sub.2 cap layer are formed in this order, and the thus formed laminate is subjected to zone melting recrystallization (ZMR) to increase the poly-Si film grain size. After the cap layer has been removed, silicon is epitaxially grown on this poly-Si layer by atmospheric pressure CVD. Thus, a polycrystalline silicon thin film 507 with a layer thickness of about 50 .mu.m and a grain size of several mm to several cm is formed. A pn junction 504 is formed in film 507 by diffusion and an anti-reflection film 505 and a surface electrode 506 are further formed to produce a polycrystalline silicon thin film cell on the surface side. Thereafter, the silicon substrate 502 and also the SiO.sub.2 insulating layer 503 are selectively removed by etching applied from the back to expose the back surface of the polycrystalline silicon thin film 507, and a back electrode 501 is formed thereon.
Thus, a polycrystalline silicon solar cell is produced through the above steps. However, the polycrystalline silicon solar cells fabricated by this process have a problem in that the energy conversion efficiency must be more improved. Moreover, according to this process, the polycrystalline silicon layer 507 and the substrate 502 are electrically insulated from each other because of the presence of the SiO.sub.2 layer between them, and hence it is necessary to carry out an etchback from the back of the substrate 502 until the SiO.sub.2 layer 502 is exposed, the SiO.sub.2 layer being further removed to expose the back surface of the polycrystalline silicon layer 507 so that the electrode material can be deposited thereon to make electrical connection. Thus, this process has another problem of complicated processing steps.